In this bog, I will explain you how the user management work in oracle 12c.

Starting from Oracle 12C there are two types of user.

Common User

Common user have the access to all PDB except seed pdb
Common user start with C## prefix
SYS and SYSTEM are the common user created by oracle
If specifying the DEFAULT TABLESPACE, TEMPORARY TABLESPACE, QUOTA and PROFILE clauses as part of CREATE USER ,then you must ensure that those objects existing in all containers (PDB) below the CDB
User-created schema objects owned by COMMON USERS cannot be shared across PDB boundaries
Common user will be visible in container as well as pluggable database
It is not recommended to create objects under common user

Local User